Field of the Invention
The present invention relates generally to Surface Enhanced Raman Spectroscopy (SERS) detectors and, more specifically, to SERS systems capable of detecting either aerosols or particulates on surfaces.
Description of the Prior Art
Low vapor pressure toxic chemicals are a recognized threat by both military and Homeland Security agencies. These agents exhibit vapor pressures on the order of 10−6 to 10−8 torr, can exist as either liquid droplets or solid particulates, and present both an inhalation threat as an aerosol and a contact threat as surface particulates/droplets. Because of their extremely low vapor pressures, these materials cannot be detected using state-of-the-art chemical agent detectors which all require the sample to be in the vapor phase for detection. Therefore, there exists a need for the capability to collect and detect the presence of these toxic aerosols and particulates on surfaces in real time. Such a system can be used as a screening device prior to a more in-depth analysis of the collected chemical aerosol/particulate, or liquid droplet.
Surface Enhanced Raman Spectroscopy (SERS) has been studied for the past 30+ years and has been demonstrated to significantly enhance the Raman cross section of molecules adsorbed to the SERS surface. There has been a large effort in developing new SERS substrates as well as understanding the mechanism of the enhancement. The SERS enhancement is primarily due to the electric field generated by illuminating the gold or silver nanostructures on the surface with a laser. The electric field is generated by the excited plasmons in the metal which, when focused in a small area, give rise to a very large electric field. The SERS enhancement is proportional to the square of the electric field and SERS studies have successfully demonstrated single molecule detection using SERS. Therefore, SERS is an extraordinarily sensitive technique and, because it reports vibrational spectroscopic information of a target, it is also highly selective.
However, sensors based on the SERS effect are not currently in use because the common sampling approaches require the dissolution of samples in an appropriate solvent followed by spotting the solution onto the SERS substrate and allowing the sample to dry. Recent efforts at producing SERS substrates within a multi-capillary environment have enabled the measurement of chemical vapors; however, this approach is not applicable for detection of chemical particulates or low vapor pressure materials.